EAGER:SUPER: In-situ Synthesis of a New Functional Material: Superconducting Polyacetylene

Syracuse University, Syracuse NY

Investigators

Abstract

Non-technical Summary Imagine a world with infinitely rechargeable batteries that are very light, store electrical energy without losses, power lines that lose no power, and portable medical imaging scanners — these are a few of the transformative applications that could be realized by the development of room-temperature (RT), ambient pressure superconductors (materials that transport electricity without resistance). Theory predicts that linear molecular conductors, like polyacetylene (PA), whose chains of carbon atoms are connected by alternating double and single bonds, might be one such superconducting material. In the laboratory, PA is anything but linear when synthesized, becoming highly disordered (like a spaghetti) and prone to reacting with itself or neighboring molecules. Just as a winding road slows traffic, what starts as a straight PA chain can twist to produce bent and twisted geometries that significantly affect its conductivity — this factor alone is a major experimental bottleneck in achieving RT superconductivity in this and other potential organic conducting polymers. With this project, supported by Division of Materials Research, Professor Michael Sponsler and his research group at Syracuse University will focus on preparing a completely new form of PA with separated and highly ordered single chains isolated in long, straight tunnels. To achieve a linear geometry capable of exhibiting potential RT superconductivity, the PA chains will be synthesized within the tunnels of honeycomb-like urea crystals, then studied to confirm their geometries and electrical behavior. If successful, the project will produce structurally ordered PA for the first time, result in a novel organic material that will have a significant impact on the theory and application of conducting polymers, and reveal the RT, ambient pressure superconductivity state, a long-standing elusive goal. Moreover, integrating a large arsenal of experimental efforts to characterize these chains and their behavior will serve to educate new chemists from undergraduate to post-doctoral levels, including women and underrepresented minorities in these interdisciplinary research projects. Technical Summary Polyacetylene (PA), the simplest conjugated polymer, is thought of as a long, straight-chain semiconductor exhibiting bond-length alternation (BLA). Based on foundational calculations predicting that PA chains cannot have BLA because the vibrational zero-point energy is above the Peierls barrier, the premise of the proposed study is that an individual, isolated chain of PA in its fully extended all-trans conformation will have a half-filled band electronic structure and thus will be a one-dimensional (1D) metallic polymer without doping. Further, the vibrational frequency of the carbon-carbon (C-C) bond alternation mode (1455 1/cm) associated with the 1D conductivity is ~7 times higher than thermal energy at room temperature (RT), precluding thermal excitation as a mechanism of electron scattering. Since there is no mechanism for resistance to the electron motion, high-temperature superconductivity in PA is a real possibility. This project, supported by the Division of Materials Research, will test the hypothesis that individual PA chains of sufficient length and isolation will show superconductivity by (1) synthesizing ordered, fully extended PA chains by encasing the polymer chains into structures with nanometer-sized diameters – the parallel tunnels of a urea inclusion crystal; and (2) characterizing the structural and electronic properties of the long chains produced to demonstrate that they show no BLA and have no measurable electrical resistance to DC currents, showing that PA acts as an RT, ambient pressure superconductor. The PA/urea inclusion crystals will be made from urea and a photoreactive precursor molecule, (E,E,E)-1,6-diiodohexatriene (DIHT), through a new light-induced process called elimination-condensation inclusion polymerization. This unique approach employs photochemical bond scission of terminal carbon-iodine (C-I) bonds from a DIHT molecule with the formation of new C-C bonds and elimination of iodine from the tunnels of the urea host crystal. The photochemical process continues until all iodine has left the crystal and high molecular weight and fully-conjugated PA chains are made within the crystalline urea tunnels. The photochemical conversion of included DIHT to PA will be monitored by mass-loss measurements to track the stoichiometric amounts of iodine lost from the initial crystal and by Raman spectroscopy that monitors the growth and disappearance of features observed in the initial stages of the process. The resulting PA/urea crystals will be studied by high-resolution X-ray diffraction, Raman spectroscopy, inelastic neutron scattering spectroscopy, and conductivity measurements, both in bulk crystal measurements and by the use of conductive atomic force microscopy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.